Digital printing involves direct printing of digital information onto a printing substrate. As shown in FIG. 1 to which reference is now made, a digital printing system 10 includes a front end computer 11 which operates to prepare the data in a format suitable for use by a printing apparatus 12 which prints the image to be printed on a printing substrate in accordance with the digital representation thereof. Front end computer 11 and printing apparatus 12 are connected by a communication cable 13.
In the prior art, front end computer 11 usually includes a composite file storage unit 14 which stores the digital representation of the images to be printed in a composite format, i.e. a CT file for each color separation to be printed and a single LW file. Usually, but not necessarily, the CT file for each separation and the single LW file represent the four process colors to be printed, namely Cyan, Magenta, Yellow and Black ("CMYB" or "CMYK").
As is well known in the art, LW files are represented in a run length encoding format, i.e. a compressed lossless format wherein pluralities of pixels having the same color are represented by a vector indicating the number of pixels having that color whereas CT files are raster files wherein the color of each pixel is independently stored.
Digital printing systems are usually capable of electronic collation, i.e. the ability to print a full document where each or at least some pages are different from each other and/or are capable to print variable information, i.e. different information on each printed copy, such as a recipient name and address. Therefore, the ability of a digital printing system to address these various needs depends on the ability of front end computer 11 to feed the printing apparatus 12 with the required data efficiently.
In operation, CPU 15 retrieves a composite file of an image to be printed from storage unit 14, transfers it to an image buffer 16 and therefrom via interface 16 and communication cable 13 to printing apparatus 12 for printing. This sequence is usually a "pipe line" sequence, i.e. while the composite file of one separation is transmitted from image buffer 16 to printing apparatus 12, the composite file representing the next separation is retrieved from storage unit 14.
Composite files are voluminous and therefore, the retrieval thereof by CPU 15 and their transmission to printing apparatus 12 is a relatively inefficient, time consuming process which adversely effects the number of prints per unit of time which digital printing system 10 provides.